USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

What characterizes diffusion-limited gas exchange in the lungs?

1. Dependent on blood flow

2. Occurs only with oxygen administration

3. Continues with pressure gradient across the pulmonary membrane

4. Caused by poor lung compliance

The correct answer is: Continues with pressure gradient across the pulmonary membrane

Diffusion-limited gas exchange is characterized by the situation where gas transfer across the alveolar-capillary membrane is primarily determined by the diffusion properties of the gas rather than the blood flow. In this context, the correct answer highlights that gas exchange continues as long as there is a pressure gradient across the pulmonary membrane. In diffusion-limited scenarios, the rate of gas transfer is limited by the ability of the gas to diffuse into the blood and not by the availability or flow of blood itself. For example, in conditions such as pulmonary fibrosis or emphysema, the thickening or damage to the alveolar membrane can impede proper diffusion, meaning that as long as there is a sufficient partial pressure difference, gas exchange will continue, albeit at a reduced rate. The other options do not accurately describe diffusion-limited gas exchange. For instance, diffusion-limited exchange is not dependent on blood flow; rather, it is impacted by the characteristics of the gas being exchanged and the integrity of the alveolar-capillary membrane. It is also not limited to conditions involving oxygen administration alone, as any gas that demonstrates diffusion limitations can fall under this category. Additionally, poor lung compliance relates more to the mechanics of breathing and does not directly relate to the diffusion characteristics of gases